Instead of assigning tasks, encourage volunteers to do work geared toward their specific interests and skill level. Give them the freedom to be innovative and creative, whether their thing is social media, data collection, baking or animal snuggles.
An occasional pizza party is great, but you want your volunteers to feel appreciated every day. It may seem obvious, but a smile and a “thank you!” go a long way. Be sure to take the time to learn their names – and make sure your staff does the same.